New concepts of transport physics in toroidal plasmas

Abstract

New concepts of transport physics in toroidal plasmas is reviewed. The transport characteristics of non-diffusion and non-locality of transport has been recognized to be important in determining radial structures of density, rotation, and temperature, as well as non-linearity in the flux-gradient relation. The non-diffusive term of momentum transport appears as a ‘spontaneous rotation and intrinsic torque’, while the non-diffusive term of particle transport appears as a ‘particle pinch and particle exhaust’. The sign and magnitude of these non-diffusive terms have been found to be sensitive to the turbulence state, which causes reversal phenomena. In the momentum transport, the spontaneous flow reversal from the co- (parallel to plasma current) direction to the counter- (anti-parallel to plasma current) direction and vice versa have been commonly observed even when the plasma parameter changes slightly. In the particle transport, especially in the impurity transport, reversals of convective radial flux from inward to outward are also observed in toroidal plasmas as phenomena of density peaking/flattening and impurity accumulation/exhaust. Non-locality of transport has been observed in the response to perturbations, such as a core temperature rise associated with the cooling at edge by pellet injection, strong coupling of transport at different radii as seen in the curvature transition of ITB, and spatial propagation of ITB regions. The turbulence with meso-scale and long correlation, which are strong candidates for causing the non-locality of the transport, have been confirmed experimentally and the coupling between the different turbulence scales has also been identified in various devices.